Process for coating metal surfaces



Patented Feb. 3, 1942 UNITED STATES PATENT OFFICE rnooass ron ooa'rmo METAL SURFACES William C. Morris, North Chicago, Ill., asslg'nor to Poor & Company, Chicago, Ill., a corporation of Delaware No Drawing. Application June 18, 1940, Serial No. 341,213

4 Claims. (Cl. 91-73) amount or degree of thermal oxidation, the invention being particularly applicable to medium firing enamels fusing in the range of approximately1260 F. to 1460 F. To that end the invention primarily has in view a new bond-dip solution of special formula introduced into the pickle line for the metal plate.

'=In the enamelling art it has been customary to increase the adherence of wet process enamels to steel by immersing the steel in an aqueous solution of nickel sulphate or, variously, nickel ammonium sulphate, buffered and controlled as to hydrogen-ion concentration by the further addition of ammonium carbonate and boric acid. Processes are known in the art whereby various other dipping solutions, such as cobalt chloride and or ammonium molybdate have been employed to accomplish a similar purpose. However, all such dips require elaborate control and are expensive, due to either the rapid exhaustion of the metal in solution or the build up of iron salts carried into the baths on the surface of the ferrous metal.

with the new bond-dip of the present application. control has been virtually eliminated, the operating range has been increased and the resultant adherence improved. Blistering, boiling or black specking and other surface defects are eliminated.

It has been determined that ferrous metal immersed in a bath composed of antimony trichloride. arsenic trichloride and stannous chloride, in solution in 18 Baum hydrochloric acid, will bond o porcelain enamel films. In the above bath the arsenic trichloride acts catalytically as a depolarizing agent to free the surface of the metal from hydrogen bubbles caused by the action of the muriatic acid on the iron, thus permittin the deposition by hydrogen overvoltage of both antimony and arsenic on the metallic surfac and as an inhibition to lessen such atack. It is necessary that the iron salts present in such a bath be maintained in a ferrous state, and a reducing agent such as stannous chloride s add d periodicallv to reduce the ferric iron for ed on or carried over on the metallic surface.

To meet the objections above specified the present invention specifically contemplates employment. for a bond-dip bath or solution, chlorides of antimony. arsenic and tin in a strongly acid solution wh ch deposits the metallic film on the immersed ferrous surface and controls the bond of the glass to the ferrous metal surface by controlling the oxidation of the same, the said bath or solution being of such a nature that a thin,

strongly adhering film of antimony and arsenic is electrochemically deposited on the ferrous metal surface immersed in the same.

While the advantages of the special bond-dip solution of the present invention are quite numerous over the usual and conventional cobalt and/or nickel dips, among themore particular advantages of the present invention are:

1. Little or no control except as to concentration is required.

2. No rinse tank required between the pickle tank and the bond-dip tank.

3. More uniform and tightly adhering bonding film is produced on ware that is tightly packed in a pickling basket.

4. Due to the high acid reserve in this type bonding solution little or no change occurs which affects the film formed and therefore the type bond produced.

In carrying forward the invention the new bond-dip solution is preferably made up according to the following formula and procedure, to wit:

First, a solution of antimony trichloride is made up by dissolving commercial antimony tetraoxide in cold 18 Baum muriatic acid in the proportion of 0.6 lb. of commercial antimony tetraoxide to each gallon of 18 Baum muriatic acid. To that solution is next added arsenic trioxlde in the ratio of one part of arsenic trioxide by weight for each 1000 parts by eight of antimony tetraoxide employed, thereby converting such arsenic trioxide into trichloride. This resultant solution will then have a yellow or amber color and thena solution of stannous chloride in strong muriatic acid is added in a sufiicient amount to turn the said prepared solution colorless or pale green. This prepared bond-dip solution is now ready for use according to the procedure described as follows:

1. Metal sheet first cleaned in a boiling alkali cleaner for 15 to 20 minutes to remove grease, oil and dirt.

2. Rinsed in a running hot or cold flowing water rinse from 1 to 5 minutes.

3. Pickled in a 610% (by weight) sulfuric acid bath for 15 to minutes at F. to F.

4. Introduced in the above described bond-dip solution.

5. Rinsing for from 1 to 5 minutes in a cold flowing water rinse.

. Neutralizing in a bath containing to 1 ounce per gallonof a mixture of 10 to 90% borax and 90 to 10% soda ash at 140 to F. for 2 to 8 minutes.

Referring to the action involved in steps 3 and 4 above noted, the action of sulfuric acid in step 3 tends to produce ferrous salts on the surface of the metal together with some ferric salts formed from scale and formed in the acid picklebath on standing idle, and appreciable amounts of these salts are carried over into the bond dip tank where ferric salts are objectionable. This condition is corrected by adding the reducing agent employed, namely stannous or bivalent tin chloride. Also, it will be noted that the effect of the arsenic trioxide is to depolarize the surface of the metal to permit the antimony to flash on by overvoltage.

An example of a glass or frit which can be i used with the above bonding method is as fol- This material is mixed, smelted, fritted and dried in the usual manner, and to produce a light grey coating the mill additions of the above will be as follows:

Parts by weight 100 Frlt Clay 7 Opacifier 3 Illmenite Black oxide iron Borax Water 40 This material is ground or milled to a fineness of -25% residue on a 200 mesh screen, preferably 4-8% residue, and the metal as previously processed in the bond-dip solution is then coated with this milled slip by spraying, dipping or slushing. Finally the coated piece is dried and fired at from 1260 to 1460 F.

In the method described the temperature employed for firing the coated metal is much lower than that employed in the conventional sheet metal enamel processes, thereby permitting the invention to be carried out in connection with lighter gauge metal sheets than heretofore possible. Accordingly, the coated metal is tired in an oxidizing atmosphere above the fusing point of the glass or enamel but at the same time below the critical point of the steel. The glasses selected for use with the bonding materials herein referred to mature at relatively medium temperaturesfrom 1260 F. to 1460 F., all of which can be bonded to the metal surface by the use of the appropriate antimony-arsenic salts or compounds.

I claim:

1. In the process of enamel-coating ferrous metals the new step of applying to the sheet a bond-coat containing an acid solution of antimony trichloride and arsenic trichloride, and stannous chloride.

2. In the process of enamel-coating ferrous metals including the new step of subjecting the sheet to a bond-dip bath containing a. muriatic acid solution of antimony trichloride, arsenic trichloride, and stannous chloride.

3. A bond-dip solution for processes of enamelcoatlng ferrous metals composed of antimony trichloride and arsenic trichloride dissolved in muriatic acid, and a reducing agent consisting of an acid solution of stannous chloride.

4. A process of preparing a bonding compound for use in enamel-coating processes which consists in first dissolving antimony tetraoxide in muriatic acid to convert the said tetraoxide into antimony trichloride, then adding to that solution a quantity of arsenic trioxide thereby converting the latter into the trichloride of arsenic and finally adding to the resultant solution an acid solution of stannous chloride.

WILLIAM C. MORRIS. 

